When an anesthetist needs a reliable, medium‑duration block without the punch of bupivacaine, Prilocaine often steps into the spotlight. It’s a versatile amide‑type anesthetic that balances rapid onset with a manageable safety profile-especially important in outpatient surgeries and obstetric procedures. This guide walks you through what Prilocaine is, how it behaves in the body, where it shines in regional anesthesia, and how to use it safely.
What is Prilocaine?
Prilocaine is a synthetic amide local anesthetic that blocks voltage‑gated sodium channels, preventing nerve depolarisation and thus pain transmission. First synthesised in the 1950s, it was introduced clinically in 1960 and quickly became popular for procedures requiring moderate duration without profound cardiotoxicity.
Pharmacology and Mechanism of Action
Like other amide‑type anesthetics, Prilocaine penetrates the neuronal membrane in its uncharged form, then binds to the intracellular portion of sodium channels. By stabilising the channel’s inactive state, it raises the threshold for action potential generation, effectively silencing peripheral nerves.
The drug’s pKa is 7.9, slightly higher than physiological pH, which explains its onset time (2-5minutes) being a touch slower than the more alkaline Lidocaine (pKa7.7) but still fast enough for most block techniques.
Key Pharmacokinetic Parameters
- Onset time: 2-5minutes for infiltration; 5-10minutes for peripheral nerve block.
- Peak effect: 10-15minutes after injection.
- Duration: 1-2hours for infiltrations; up to 3hours for nerve blocks when used at 4% concentration.
- Metabolism: Primarily hepatic via N‑dealkylation to o‑toluidine, a metabolite implicated in methemoglobin formation.
- Excretion: Renal elimination of unchanged drug and metabolites; half‑life ≈ 1.5hours in healthy adults.
Prilocaine in Regional Anesthesia Techniques
Regional anesthesia encompasses any technique that numbs a specific area by injecting anesthetic near nerves or the spinal cord. Prilocaine’s sweet spot is medium‑duration blocks where rapid recovery of motor function is desirable.
Peripheral nerve block applications include:
- Upper‑extremity blocks (interscalene, supraclavicular, infraclavicular) for hand surgery.
- Lower‑extremity blocks (femoral, sciatic, popliteal) for ankle or foot procedures.
Because Prilocaine’s cardiotoxicity is lower than that of bupivacaine, it is a safer option for patients with cardiac risk factors. In obstetrics, a 2% Prilocaine solution is approved for epidural analgesia during labour, offering adequate pain relief while limiting motor block, which helps mothers stay mobile.
Epidural anesthesia using Prilocaine is favoured for short‑to‑mid‑length deliveries. A typical regimen involves a 5-10ml loading dose of 2% Prilocaine, followed by a maintenance infusion of 4-6ml/h.
Safety Profile and Methemoglobinemia Risk
The most discussed adverse effect of Prilocaine is its potential to cause methemoglobinemia, a condition where iron in haemoglobin is oxidised to the ferric (Fe³⁺) state, impairing oxygen delivery. The risk correlates with dose and patient factors (e.g., G6PD deficiency, infants under 6months, or concomitant use of oxidising agents).
Clinical data suggest that doses above 600mg (approximately 8mg/kg for adults) markedly increase the incidence of methemoglobin levels >5%. For most regional blocks, the total dose stays well below this threshold, making the risk manageable.
When methemoglobinemia does occur, treatment is straightforward: intravenous methylene blue (1-2mg/kg) reverses the oxidation, and supplemental oxygen helps until haemoglobin function normalises.
Comparing Prilocaine with Other Common Local Anesthetics
| Agent | Onset (min) | Duration (h) | Max Safe Dose (mg/kg) | Methemoglobin Risk |
|---|---|---|---|---|
| Prilocaine | 2-5 | 1-3 | 6 | Moderate (dose‑dependent) |
| Lidocaine | 1-3 | 1-2 | 7 | Low |
| Bupivacaine | 5-10 | 4-8 | 2.5 | Very Low |
| Articaine | 1-2 | 1-2 | 7 | Low |
From the table you can see why Prilocaine sits between Lidocaine and Bupivacaine. It offers a slightly longer duration than Lidocaine without the prolonged motor block of Bupivacaine, and its cardiotoxicity profile is more forgiving than bupivacaine’s.
For clinicians who need a block lasting up to three hours but want quick return of limb function, Prilocaine is often the agent of choice.
Practical Tips for Clinicians
- Dosage calculation: Always base the total milligram limit on body weight. For a 70kg adult, stay under 420mg for infiltrations.
- Mixture strategies: Combine 2% Prilocaine with 0.5% Bupivacaine to prolong analgesia while limiting each drug’s individual toxicity.
- Monitoring methemoglobin: Use a co‑oximeter if the patient receives >400mg or shows cyanosis unresponsive to oxygen.
- Special populations: Reduce maximum dose by 25% in elderly or patients with hepatic insufficiency.
- Adjuncts: Adding epinephrine (1:200,000) prolongs block duration by up to 30% and reduces systemic absorption, thereby lowering methemoglobin formation.
Remember that technique matters as much as drug choice. Accurate needle placement, aspiration before injection, and incremental dosing help minimise systemic peaks and improve block quality.
Frequently Asked Questions
What makes Prilocaine different from Lidocaine?
Prilocaine has a slightly higher pKa, leading to a marginally slower onset, but it provides a longer duration (up to 3hours) and a lower risk of cardiotoxicity. Its main drawback is the dose‑related risk of methemoglobinemia, which is minimal at routine clinical doses.
Is Prilocaine safe for use in obstetric epidurals?
Yes. A 2% Prilocaine solution is approved for labour epidurals. It offers adequate analgesia while preserving motor function, allowing the mother to move and change positions during delivery. Doses are kept well below the methemoglobinemia threshold.
How do I recognize methemoglobinemia after a Prilocaine block?
Look for cyanosis that does not improve with supplemental oxygen, chocolate‑brown blood, and a drop in pulse oximeter reading while arterial blood gas shows a normal PaO₂. A co‑oximeter will confirm elevated methemoglobin levels.
Can I mix Prilocaine with other local anesthetics?
Mixing is common practice. A typical blend is 2% Prilocaine with 0.5% Bupivacaine in a 1:1 ratio, delivering the rapid onset of Prilocaine and the prolonged duration of Bupivacaine while keeping each agent below its toxic ceiling.
What patient factors increase the risk of methemoglobinemia?
Infants under 6months, patients with glucose‑6‑phosphate dehydrogenase (G6PD) deficiency, those on other oxidising drugs (e.g., dapsone, sulfonamides), and individuals with severe hepatic disease are more susceptible. Dose reduction or alternative agents are advised for these groups.
